Advice on a flat to pitch roof conversion

[CortinaV8]

Hey,

I'm after a bit of advice on this project.

My garage is 3.5x10m and runs the length of the house, currently it has a flat roof that I have been patching up forever, the time has come to re-roof and I would rather change to a lean to pitched type roof rather than just replace with another felt/fibreglass roof.

The wall construction is solid brick, two skins on proper foundations.

I was going to use Marley Thrutone composite slate (600x300) laid at somewhere around 15-17 degrees.

Where I want some advice is on the timber sizes to be using. Looking at span charts I have come up with the following.

Rafters - C16 47x150 (span 3.55m @ 17 degrees, 400 centres)
Ceiling Joists - C16 47x150 (span 3.35m, 400 centres)

What should I be using for Joist and Rafter plates, I assume I am right @ 47x100 C16 for the wall plate.

I've added a photo, if anyone has any suggestions or notice any potential pitfalls that would be great.

 

[catlad]

6x2 should be be fine for what you have in mind.

 

[noseall]

Have you considered a hip design?

Have you considered soffit and gutter overhang on the boundary elevation?

150mm x 50mm may be on the limit with the rafters and probably falls short on the joists. 175mm is more likely.

Your joists can either be let into the wall or secured to a hanging plate of the same size as the joists, which would need bolting to the wall. Use jiffy hangers if you go with a joist hanging plate.

The rafter pole plate only need be thick enough to fix to i.e. if it is located below the rafter and that rafter abuts the wall then the pole plate will only be stopping it moving sideways and upwards (uplift). You may need to either notch the rafter top cut onto the pole or use truss clips to prevent uplift. We would use the same timber as the rafter for the pitching (pole) plate. Bolt the pole plate to the wall.

The joists will require central bridging along with straps into the gables (if you go for straight lean-to). Similarly the rafters will need strapping to the gable also. Straps to carry over three joists/rafters with solid bridging between etc.

The wall plate will be standard tanolised 75mm x 100mm bedded on mortar and strapped every 2m.

Don't forget a loft hatch.

I'd also consider fitting the joists at house ceiling height and making them suitable for carrying a floor load.

Don't forget to inform BC.

 

[CortinaV8]

Cheers Noseall, very informative.

I have considered a hip design, a few of my neighbours have done the same. I'm pretty confident in going ahead with a lean to design but wouldn't really know where to start with a hipped roof to be honest.

Soffit/Gutter overhang is onto a footpath, I was intending to mirror what my neighbour directly opposite had done, his was approved/signed off by BC a couple of years ago so can't see an issue.

I was intending on using hangers for the floor joists and notching the rafters onto the pole/plate as well as truss clips at both ends.

I hadn't considered the strapping of the joists/rafters at the gables to stop lateral movement, seems pretty obvious now I think about it.

I need to chase up building control again, so far they have been worse than useless! I'm getting to the point where I am just going to have to start the project and they will have to come and inspect it later.

 

[freddiemercurystwin]

Unless you actually appoint BC then what do you expect? They're not an advisory service, its not within their obligations to advise, though many do.

 

[CortinaV8]

I have submitted the application and paid the fee, probably three weeks ago now. Someone is on holiday and there is a backlog, apparently...

My councils planning department don't give out free advice, I did try that avenue first.